Semiconductor integrated circuit composed of cascade connection of inverter circuits

ABSTRACT

A semiconductor device composed of cascade connected inverter circuits each comprising a load depletion type MIS transistor and a driving enhancement type MIS transistor. The semiconductor device can be properly operated by setting the threshold voltage of the load MIS transistors at a predetermined value, by selecting the dimensions and materials thereof.

United States Patent Masuhara et al.

4 Oct. 24, 1972 [54] SEMICONDUCTOR INTEGRATED CIRCUIT COMPOSED OF CASCADE CONNECTION OF INVERTER CIRCUITS Inventors: Toshiaki Masuhara, Tokorozawa;

[51] IhL C I, .Q ...H0ll 19/06 [58] Field of Search ..317/235 B, 235 G; 307/205, 307/251, 279, 304; 330/35 Primary Examiner-Jerry D. Craig [72] Art C & Antonell Minoru Nagata, Kodaira; Masharu omey tag I Kubo, Hachioji, all Of Japan 57 ABSTRACT [73] Assignee: Hitachi, Ltd., Tokyo, Jap A semiconductor device composed of cascade con- [22] Filed: May 24, 1971 nected inverter circuits each comprising a load depletion type MIS transistor and a driving enhancement [21] Appl' N05 @154 type MIS transistor. The semiconductor device can be properly operated by setting the threshold voltage of [30] Foreign Application Pri ri Data the load MIS transistors at a predetermined value, by

selecting the dimensions and materials thereof.

May 27, 1970 Japan ..45/44892 2 Claims, 5 Drawing Figures [52] U.S. Cl. ..3l7/235 R, 307/304, 317/235 G,

r V0 t 7 t BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a monolithic integrated circuit, and more particularly to an integrated circuit including a plurality of inverter circuits each comprising a driving field effect type transistor and a load field effect type transistor.

2. Description of the Prior Art An inverter circuit has heretofore been proposed which employs an enhancement type MOS (metal oxide semiconductor transistor as a driving field effect type transistor and a depletion type MOS transistor as a load field effect type transistor. Such an inverter circuit is superior to an inverter circuit which employs enhancement type MOS transistors for both driving and load field effect transistors in that the voltage efficiency is higher, the transient response is faster and the source voltage can be made lower because the impedance of the load MOS transistor is lower.

Such inverter circuits are seldom used individually, but usually used in combination as, for example, a memory circuit or logic circuit. However, when the component elements composing the memory circuit or logic circuit have uneven characteristics, the memory circuit or logic circuit does not operate properly. Thus, although it has been known that an improved inverter circuit can be composed of an enhancement type MOS transistor and a depletion type MOS transistor, it has not been known before how to construct the elements of a circuit composed of a plurality of such inverter circuits to properly operate the circuit.

SUMMARY OF THE INVENTION An object of the present invention is to provide a semiconductor device which comprises a plurality of inverter circuits and which operates properly.

In brief, the semiconductor device according to the present invention comprises a depletion type MIS (metal insulator semiconductor) transistor functioning as a load and an enhancement type MIS transistor functioning as a driver, the dimensions of the transistor and the material and thickness of the insulating film being .selected so that the transistor has predetermined characteristics.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is the circuit diagram of an embodiment of the V, of 5 volts. The output voltage 0.5 volt of the first ina DESCRIPTION OF THE PREFERRED EMBODIMENTS Refering to FIG. 1 which shows a circuit diagram of a pair of cascade connected inverter circuits, a first inverter comprises a driving enhancement type MIS transistor T, and aload depletion type MISEtransistor T11 and a second inverter similarly comprises a driving enhancement type MIS transistor Tag and a load depletion type MIS transistor T An input signal V, is supplied to an input terminal 1. The output signal of the first inverter is supplied through a junction point 2 to the second inverter, and the output signal-V, of the Referring now to FIGS. 2a and 2b, in a semiconduc-' tor substrate 21 having one conductivity type (for example, a P conductivity type silicon substrate) impurity diffused regions 22, 23 and 24 having an opposite con-' ductivity type (for example, N conductivity type) are formed. The region 22 serves as the drain region of a load MOS transistor, the region 23 serves as the source region of the load MOS transistor and, at the same time, as the drain region of a driving MOS transistor, and the region 24 serves as the source region of the driving MOS transistor. An insulating layer 25, for example, an SiO layer, is formed over the MOS transistors except for the portions where electrodes 28, 30 and 32 are provided. An insulating layer 26, for example, an AI O layer, is formed over-the exposed surface of the semiconductor substrate 21 and the insulating layer 25 except for the portion where the gate electrode 29of the load MOS transistor is provided. The gate electrode 31 of the driving MOS transistor is provided on the insulating layer 26. An insulating layer 27, for example, an SiO 2 layer, is formed over the exposed surface of the insulating layer 26. The electrode 30, which is common to the source electrode of the load MOS transistor and the drain electrode of the driving MOS transistor, is connected with the gate electrode 29 of the load MOS transistor. The electrode 28 is the drain electrode of the load MOS transistor.

In this manner a load depletion type MOS transistor and a driving enhancement type MOS transistor are formed. The relation between the input V, and the output V,, of the thus formed inverter circuit varies greatly, to an extent depending on the threshold voltage K of the load depletion type MOS transistor as shown in FIG. 3, where the source voltage V is set at 5 volts. Consequently, although a plurality of inverter circuits having certain characteristics can be connected to form a proper memory circuit or logic circuit, a memory or logic circuit formed of those having other characteristics does not properly operate. For example, at a threshold voltage V of 2 volts the output voltage of the first inverter circuit is 0.5 volt for an input voltage verter circuit is an input to the second inverter circuit,

the output .V, of which volts. Thus, there is no loss of the input signal relative to the output signal. Such converters properly operate even if connected in multiple stages. However, if the threshold voltage Kn of the inverter circuits is 5 volts, the output voltage of the first inverter circuit is about 3 volts for an input voltage V, of 5 volts, and the output voltage V of the second inverter circuit is 4.5 volts. Thus, the inverters do not operate properly in that the output voltage is low relative to the input voltage, from which it is clear that a proper output cannot be obtained relative to an input when such inverter circuits are connected in multiple stages. Consequently, to construct a predetermined circuit by connecting inverter circuits in multiple stages it is necessary to construct the circuit out of transistor elements having a predetermined threshold voltage Kg LI and W1 are the length and width, respectively,

of the channel in the transistor as shown in FIG.

2b, e e 6,, are the dielectric constants of the gate insulator layers, T T T are the thicknesses of the gate insulating layers, and m is the mobility in the channel. The current 1,, which flows through the driving enhancement type MIS transistor is where V, is the input voltage and V is the output voltage. If the inverter circuit is in an on-condition, V, V,,, V,,. Therefore, Equation (2) becomes I e n (3) Where L,, and W1 are the length and width, respectively, in the channel of the transistor as shown in FIG. 2b, 6,, e e, are the dielectric constants of the gate insulator layers, T T T,,' are the thickness of the gate insulator layers, #4 is the mobility in the channel, and V is the threshold voltage of the driving MIS transistor.

as follows:

From the fact that inthe inverter circuit .1 1 andthe highest level of the input signal V, (the output voltage when the inverter of the preceding stage is in an off state) is approximately equal to the source voltage V i.e., V V the output voltage V,, when the inverter is in an on state is expressed fromEquations (1) and (3) of the second inverter circuit becomes the same level of signal as .V,,, the following relation should be satisfied:

V0 ld From relations (4) and (5) it follows that 1v.1| /f-FV'Z'v.. I m (6) Consequently, by selecting the dimensions and materials of the load and driving MIS transistors so thatthey satisfy the relation (6), the circuit composed of such transistor elements can always be operated stably.

As described above, the circuit operates better by setting the absolute value of the threshold voltage Vt of the load depletion type MIS transistor at a value .equal to or lower than a predetermined value. However, the threshold voltage Vn varies depending on the voltage at the output terininal of the inverter circuit. This variation av is expressed by Av.1= 15H W. 1 2 n where q-is the electronic charge, e, is the dielectric constant of the semiconductor substrate, and .N is the impurity concentration in the semiconductor substrate.

When the variation AV, is larger than JV I, the load MIS transistor operates in an enhancement mode and no longer operates in a depletion mode. That is, as is shown by the input V, versus output V characteristics in FIG. 4, the ofi-level of the inverter circuit is sufficiently high (equal to the source voltage Y =5 volts) when 'V, =1 volt, but lowers (about 2.6 volts) when V ?).5 volt, not to satisfactorily operate though ha mal-functioning. Consequently, in order to improve the transient response with a low impedance, the threshold voltage ,V of the load MIS transistor is determined so that tlfi relation l ul u 6 gate electrode, and a source electrode connected to W1 and W are the widths of the channels in said said gate electrode, and operating in the saturation redepletion and enhancement mode transistors, gion of the drainvoltage-drain current characteristic respectively; thereof, and an enhancement mode MIS transistor hav- V is the source voltage, ing a drain electrode connected to said source elec- 5 V is the threshold voltage of said enhancement trode of said depletion mode transistor, a gate elecd tra si t r, trod? Connected to anvinput terminal for receiving an e e ..-.....e,, are the dielectric constants of the gate input signal, and a source electrode connected to a insulator layers f id d l i d M13 constant bias source, and operating in the triode region transistor, of the drain voltage-drain current characteristic thereof T1, "T" are h hi k f the gate insulator when an input signal is supplied to said input terminal, 4 layers f Said depletion mode transistor and wherein the dimensions of the transistors are are the thicknesses ofthe gateinsula selected so that the channel conductance [3 and B; of each MIS transistor satisfies the following relationship with respect to the threshold voltage V of the deplel5 tor layers of said enhancement mode MlS transistor, and m and a are the mobilities in the channels of said tion mode MIS transistor: depletion and enhancement mode MIS transistors,

respectively. E [VH1 2. A semiconductor device according to claim 1, in B1 d td) d which said threshold voltage of said depletion Where W 1 mode MlS transistor furthersatisfies the relation:

B1=- W T T T Ll )x q B a 6 61 n VVEI e2 ,.s., r W 1 where L 1 2 1+ 2i Md qis the electron charge,

6 e ,1 e, is the dielectric constant of the semiconductor subi strate, and L1 and Ld are the lengths of the channels in Said N1s the impurity concentration inthe semiconductor depletion and enhancement mode transistors, respec- Substrate tively, 

1. A semiconductor integrated circuit comprising a plurality of cascade connected inverter circuits each comprising a depletion mode MIS transistor having a drain electrode connected to a bias voltage source, a gate electrode, and a source electrode connected to said gate electrode, and operating in the saturation region of the drainvoltage-drain current characteristic thereof, and an enhancement mode MIS transistor having a drain electrode connected to said source electrode of said depletion mode transistor, a gate electrode connected to an input terminal for receiving an input signal, and a source electrode connected to a constant bias source, and operating in the triode region of the drain voltage-drain current characteristic thereof when an input signal is supplied to said input terminal, and wherein the dimensions of the transistors are selected so that the channel conductance Beta d and Beta l of each MIS transistor satisfies the following relationship with respect to the threshold voltage Vt of the depletion mode MIS transistor:
 2. A semiconductor device according to claim 1, in which said threshold voltage Vt of said depletion mode MIS transistor further satisfies the relation: where q is the electron charge, epsilon s is the dielectric constant of the semiconductor substrate, and N is the impurity concentration in the semiconductor substrate. 